Radio-frequency excitation arrangements often consist of a radio frequency generator (RF generator) with an internal reactance network and a matching network to which a load can be connected. The load may be, e.g., the gas discharge in a plasma chamber. While plasma processes are performed in the plasma chamber for coating and processing materials, sparkovers, so-called arcs, occur again and again in the plasma chamber. These processes can cause the energy stored in blind elements of the matching network, the reactance network, or in other parts of the radio frequency excitation arrangement, to be rapidly discharged. The blind portions of the current, the voltage, and the power may amount to a multiple of the real portions and depend on the quality factor of the networks, and therefore far exceed the load limit of the overall system. The discharged energy, in particular the associated high voltages, can permanently damage or predamage the component assemblies and components, which will cause immediate or later failure of the RF generator or of the matching network.
MOSFETS are often used in transistorized RF amplifiers, which are part of the RF generator. MOSFETS are particularly sensitive to excess voltages. An excess voltage (voltage above the admissible voltage) from drain to source can cause a so-called avalanche effect that can destroy a MOSFET within a very short time. In the avalanche effect, electrons are accelerated by the high voltage such that they release further electrons from the lattice of the semi-conductor of which the MOSFETs are produced. The further electrons are also accelerated causing a chain reaction that can result in local breakdowns and degeneration of the lattice structures, generating so-called “hot spots”. The MOSFET is destroyed within a relatively short time, ranging from a few μs to a few days, the time depending on the structure of the MOSFET, the voltage applied to the MOSFET, and the temperature at which the MOSFET is operating.
To avoid this problem, the power discharged to the load and the reflected power are measured in an RF system in different ways. Often, power control is used to keep the voltage at the sensitive components, i.e., in particular, at the MOSFETs within the permitted limits. This power control can be, however, much too slow for short-term pulses.
There have been attempts to keep disturbances by the load, i.e., in particular, a plasma source, away from the sensitive components using suitable filter arrangements. For example, U.S. Pat. No. 5,747,935 discloses a circuit for suppressing undesired disturbances from plasma sources using a filter that absorbs energy at all frequencies except for the basic frequency. Very fast pulses that occur during arcing in the plasma chamber, mainly those which have slew rates in the range of the slew rates of the basic frequency, are not absorbed thereby or are absorbed only to an insufficient extent. A switching device of this type can be realized for RF generators with a very small variation of the basic frequency. Moreover, a switching device of this type can represent an additional undesired energy storage.
The components of a radio-frequency excitation arrangement can be overdimensioned. For example, the MOSFETs that are used can bear an excess voltage of 600V even though only voltages of up to 300V are applied during normal operation. MOSFETs of this type are typically several times more expensive and can have a high forward resistance that can have a negative effect on the efficiency of the RF generator.
A so-called clipper circuit can be used to limit the voltage at sensitive components. Clipper circuits are, e.g., suppressor diodes, Zener diodes, or similar components or circuit arrangements consisting of several components. The reverse resistance of these clipper circuits changes at a defined voltage, the so-called breakdown voltage. When these components have adequate dimensions, they can protect sensitive components in the RF excitation arrangement. It may not be possible to exactly adjust these clipper circuits to the maximum admissible breakdown voltage at the component to be protected. If the voltage reaches a value close to the breakdown voltage, individual small breakdowns, noise, and harmonics can be generated. Such clipper circuits also have a capacitance that can generate a loss at the provided RF power. The RF excitation arrangement is changed by noise, harmonics, and capacitance mostly with a negative result. Such clipper circuits also can have a long uncontrollable recovery time that can result in instabilities in the RF generator.